Filtering septums are used in a variety of industrial filtration processes and it is particularly well adapted for filtration in a pressurized vessel for treatment of waste water and the like. In pressure vessel work the septum divides a container into two compartments. A fluid contaminated with solid particles is directed into one of the compartments which is filled with a highly porous substance that absorbs or separates the solid particles from the fluid. As an example, the absorption medium which filters the fluid is activated charcoal. The filtering septum permits the passage of the filtered liquid, the filtrate, but its holes are sufficiently small to retain the activated charcoal.
An edge-filtering septum is taught by U.S. Pat. No. 3,052,360 to Imershein which consists of a pair of superposed secured sheets of metal each of which is provided with a multiplicity of punched holes arranged in a definite spatial pattern in order that the sheets may be so relatively positioned that none of the holes in one sheet is in registry with any of the holes in the other sheet. The medium being filtered is not permitted to flow directly through the septum. Rather, after flowing through the holes in one sheet, it must flow edgewise between the sheets to reach the holes in the other sheet. Hence, the term "edge" filtration.
The Imershein patent solved two major problems in the manufacture of edge filtering septums. The first problem was the unpredictable stretching of, and other dimensional deviations in, a metal sheet when it is subjected to the manufacturing operation which creates the multiplicity of holes therein. It was previously virtually impossible to position two such sheets, in superposed relation, so that all the holes of one would be out of registry with all the holes of the other. This problem was especially acute since the diameters of the holes involved are only a few thousandths of an inch, and the center-to-center distances between adjacent holes is of the order of one-tenth of an inch. Therefore, in a sheet 8 feet long (for example) and two feet wide, over 200,000 such holes are present. Obviously, in such circumstances even a slight difference between the amounts of dimensional deviation caused in the two sheets by the hole-forming operation might render it impossible to so arrange the sheets that all the holes of one coincide only with the unperforated areas of the other. If there is any alignment at all between any holes in the respective sheets, the edge filtering effect can be seriously affected.
The second problem arose from the fact that the liquid being filtered passes edgewise between the sheets, and hence variations in spacing between the sheets creates variations in filtering results. If it is attempted to overcome this problem by pressing the sheets together more tightly, no space at all might remain to accommodate the flow. If, on the other hand, the pressure holding the sheets together is reduced, variable spacing and consequent non-uniform flow through action might be permitted. Any appreciable increase in the spacing between the sheets, of course, cancels the advantages of edge filtration.
In the prior art septum the punching operation resulted in the formation of a rough edge commonly referred to as a "burr" on the inner face of the top sheet about the periphery of each hole. These burrs bear against unperforated areas in the lower sheet after the sheets are shifted to bring the holes out of registry and generally define the distance between the two sheets.
The prior art septum offers significant improvements since it is able to more effectively retain the medium that performs the filtration than screen and cloth type filters of comparable hole size, because of the "edge-filtration" effect. However, it has been observed that under increased pressure, the prior art septum will exhibit a tendency to permit a greater passage of liquid through its central regions than throughout the peripheral portions. This uneven flow distribution is known as "channeling". While channeling does not necessarily affect the quality of the filtration, it will cut down the efficiency of the process because the activated charcoal will not be utilized uniformly. Hence, the activated charcoal will have to be replaced faster than it might otherwise.